North Atlantic Treaty Organization (NATO) is a military alliance consisting of the United States, Canada, and 14 other Western countries. The 14 countries are Belgium, Denmark, France, Germany, Greece, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, Spain, Turkey, and the United Kingdom.

Formed in 1949, NATO was set up largely to discourage an attack by the Soviet Union on the non-Communist nations of Western Europe. After World War II ended in 1945, an intense rivalry had developed between Communist countries, led by the Soviet Union, and non-Communist nations, led by the United States. This rivalry became known as the Cold War. In 1955, the Soviet Union and Communist nations of Eastern Europe formed their own military alliance to oppose NATO. The Soviet-led alliance was called the Warsaw Pact.

NATO also was established to keep the peace among former enemies in Western Europe. In World War II, for example, Italy and Germany had fought most of the other countries that later became NATO members.

In forming NATO, each member country agreed to treat an attack on any other member as an attack on itself. Militarily, the United States was -- and still is -- the alliance's most powerful member, in part because of its large supply of nuclear weapons. The NATO countries believed the Soviet Union would not attack Western Europe if such an attack would trigger war with the United States. NATO's policy is known as deterrence because it is designed to deter (discourage) an attack. NATO's purpose, however, has been less clear since the Warsaw Pact and the Soviet Union were dissolved in 1991.

NATO has a civilian branch and a military branch. The civilian branch includes the North Atlantic Council, the highest authority in NATO. The council consists of the heads of government of the NATO members or their representatives. A secretary-general heads the council. A European has always held this post. Decisions of the council must be unanimous.

NATO's military branch includes three commands: Allied Command Atlantic, Allied Command Channel, and Allied Command Europe. Allied Command Europe has traditionally functioned as the heart of NATO. Its commander has always been a U.S. general. NATO's military commanders report to the Military Committee, which reports, in turn, to the North Atlantic Council. The Military Committee consists of the military chiefs of staff or other representatives of the NATO nations.

NATO was formed as a result of the North Atlantic Treaty, which was signed by 12 countries on April 4, 1949, in Washington, D.C. The 12 countries were Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom, and the United States. Greece and Turkey joined NATO in 1952. West Germany joined in 1955. Germany replaced West Germany as a NATO member in 1990, when West Germany and East Germany were united. Spain joined NATO in 1982.

During the Cold War, NATO helped maintain peace in Europe through its policy of deterrence. But it also experienced disagreements among its members. The most troublesome involved nuclear weapons. United States officials generally insisted that NATO rely on nuclear weapons to deter a Soviet attack. Some people in NATO countries, however, opposed the use of these weapons. Also, European countries occasionally doubted that the United States would actually use nuclear weapons to defend Europe. Their doubts were based on the fact that the Soviet Union also had a powerful nuclear force. For these reasons, Britain and France built their own nuclear weapons. In 1966, France pulled its troops out of the NATO military command, though it remained a NATO member. Before France withdrew its troops, NATO's central office had been in Paris. In 1967, the organization moved its headquarters to Brussels, Belgium.

NATO's biggest crisis followed the breakup of the Warsaw Pact and the Soviet Union in 1991. The Soviet Union broke apart into a number of independent states. Most of these states -- and the Soviet Union's former allies in Eastern Europe -- rejected Communism. Some people felt that without its traditional Communist enemies, NATO had lost its purpose and should disband.

Some NATO leaders proposed offering membership in NATO to such former Warsaw Pact lands as Poland, Hungary, Ukraine, and the Czech Republic. Other NATO leaders worried that offering membership to former Soviet allies might lead to a dangerous conflict with Russia.

To help resolve the uncertainty about NATO's future, the alliance began the Partnership for Peace program in 1994. Over 20 countries joined the program, including Russia. Most of the other countries that joined were Eastern European nations. The program provides for joint military planning and exercises with NATO members but does not involve formal NATO membership.

In the mid-1990's, NATO took military action against Bosnian Serb forces to help end a civil war in the former Yugoslav republic of Bosnia-Herzegovina. The Bosnian Serbs were fighting Bosnia-Herzegovina's government. NATO's action increased tension between NATO and Russia, a traditional ally of the Serbs. In late 1995, the Bosnian government and the Bosnian Serbs agreed to a peace treaty, and NATO troops soon replaced United Nations troops as the peacekeeping force in Bosnia.

In 1997, Russia announced that it would not oppose the eastward expansion of NATO. That same year, NATO and Russia agreed to form a joint council to give Russia a voice in NATO decision making. Later in 1997, NATO invited the Czech Republic, Hungary, and Poland to seek formal NATO membership. All three countries had expressed interest in becoming members. NATO decided not to offer membership to a number of other Eastern European countries.